The structure of a sensory relay will be studied in terms of the detailed synaptic relationships established within architectonically distinct regions by functionally different systems. Light and electron microscopical studies of normal tissue impregnated by the Golgi methods or prepared by a method that allows identifiable processes to be filled with horseradish peroxidase will be undertaken of the lateral geniculate nucleus in cats, ferrets and monkeys. In addition the development of the normal nucleus will be studied in cats and ferrets and the structure of axonal sprouts, which can be induced to invade "foreign" territory, will be studied with the new horseradish peroxidase method. In this way the early growth of nerve fibers into their specific terminal fields can be studied, and the extent to which this growth can be modified by experimental manipulations can be displayed.